When a silicon (Si) substrate is exposed to the atmosphere, a thin natural oxide film is formed on the surface. If the natural oxide film is formed on the silicon substrate, an abnormal growth of a silicon nitride (SiN) film occurs when forming the silicon nitride film on the silicon substrate. In order to prevent this, a technique has been known in which, after the silicon substrate is conveyed to a reactor, a thin base film formed of a silicon oxynitride (SiON)-based thin film having small substrate surface dependence is formed on the silicon substrate, and then, a silicon nitride-based thin film is formed thereon in succession. See, for example, Japanese Patent Laid-Open Publication No. 07-245268.